Operation of lean burn engines, e.g., diesel engines and lean burn gasoline engines, provide the user with excellent fuel economy, and have very low emissions of gas phase hydrocarbons and carbon monoxide due to their operation at high air/fuel ratios under fuel lean conditions. Emissions of diesel engines include particulate matter (PM), nitrogen oxides (NOx), unburned hydrocarbons (HC) and carbon monoxide (CO). NOx is a term used to describe various chemical species of nitrogen oxides, including nitrogen monoxide (NO) and nitrogen dioxide (NO2), among others.
There are major differences between catalyst systems used to treat diesel engine exhaust gas and gasoline engine exhaust gas. A significant difference between the two types of engines is that gasoline engines are spark ignited and operate within a stoichiometric air to fuel ratio, and diesel engines are compression ignition engines that operate with a large excess of air. The emissions from these two types of engines are very different and require completely different catalyst strategies. Generally, the treatment of diesel emissions is more complicated than gasoline engine emissions treatment. In particular, gasoline engines produce exhaust in which the soluble organic fraction (SOF) is lower. The SOFs in diesel emissions is a combination of unburned diesel fuel and lubricating oils.
For diesel engines traditionally, platinum-based DOC catalysts have been used. It is also known that hydrocarbon trapping materials such as zeolites are effective for treating diesel exhaust. For example, U.S. Pat. No. 6,093,378 (Deeba) is directed to a diesel exhaust catalyst, providing two or more zeolites and a supported precious metal component, such as platinum. A first zeolite is provided in the substantial absence of precious metals and a second zeolite is doped with a precious metal. In this way, the first zeolite adsorbs gaseous hydrocarbons and the second zeolite catalyzes the NOx reduction.
Palladium, however, has proven to be suitable in DOC catalysts in conjunction with platinum to reduce the required amount of platinum, despite it being somewhat less reactive on a weight basis. Due to the lower reactivity of palladium in DOC catalysts, it is important to ensure that it is located in the DOC catalyst in a way that does not inhibit its performance.
As emissions regulations become more stringent, there is a continuing goal to develop diesel oxidation catalyst (DOC) systems that provide improved performance, for example, light-off performance. There is also a goal to utilize components of DOCs, for example, the zeolites and palladium, as efficiently as possible.